Among information recording mediums having a recording layer utilizing the property of magnetism, light and magneto-optics, there is a magnetic disk as a typical one. Conventionally, as a substrate for a magnetic disk, an aluminum substrate has been widely used. However, recently, in accordance with a request to reduce an amount of float-up of a magnetic head in order to increase a recording density, the ratio of employing a glass substrate as a substrate for a magnetic disk increases due to the reason that since the glass substrate is excellent in surface flatness in comparison with the aluminum substrate and has few surface defects, an amount of float-up of a magnetic head can be reduced by the use of the glass substrate.
Such a glass substrate for an information recording medium like a magnetic disk is produced by a step of applying a polishing process on a glass substrate called a blank material. As a method of producing a glass substrate (a blank material), a method of producing a glass substrate by pressing shaping a molten glass and a method of producing a glass substrate by cutting a glass plate produced by a float method are well known. Among these methods, a method of producing a glass substrate by directly pressing shaping a molten glass has received specifically a lot of attention, because high productivity is expected from this method.
However, in the method of producing a glass substrate by pressing shaping a molten glass, since the flatness of a produced glass substrate is bad, it is necessary to conduct an annealing process in order to correct the flatness. Therefore, there is a problem to need a lot of time and labor for a post processing.
In order to improve the deterioration of a flatness due to a warp of a glass substrate, a pressing shaping method is proposed to employ a mold in which a receiving mold, or a opposing mold or at least a part of a shaping surface of a receiving mold or a opposing mold is subjected to a heat insulation processing (refer, for example, Patent document 1). Further, as the heat insulation processing, Patent document 1 proposes a method of making a shaping surface to a rough surface.    Patent document 1: an official report of Japanese Patent Unexamined Publication No. 10-194763